Respiratory syncytial virus (RSV) is an enveloped non-segmented negative-strand RNA virus in the family Paramyxoviridae, genus Pneumovirus. It is the most common cause of bronchiolitis and pneumonia among children in their first year of life. RSV also causes repeated infections including severe lower respiratory tract disease, which may occur at any age, especially among the elderly or those with compromised cardiac, pulmonary, or immune systems. Passive immunization currently is used to prevent severe illness caused by RSV infection, especially in infants with prematurity, bronchopulmonary dysplasia, or congenital heart disease. Current treatment includes administration of a RSV-neutralizing antibody, Palivizumab (SYNAGIS®; MedImmune, Inc.), which binds a 24-amino acid, linear, conformational epitope on the RSV Fusion (F) protein.
In nature, the RSV F protein is initially expressed as a single polypeptide precursor, designated F0. F0 trimerizes in the endoplasmic reticulum and is processed by a cellular furin-like protease at two conserved sites, generating, F1, F2 and Pep27 polypeptides. The Pep27 polypeptide is excised and does not form part of the mature F protein. The F2 polypeptide originates from the N-terminal portion of the F0 precursor and links to the F1 polypeptide via two disulfide bonds. The F1 polypeptide originates from the C-terminal portion of the F0 precursor and anchors the mature F protein in the membrane via a transmembrane domain, which is linked to an ˜24 amino acid cytoplasmic tail. Three protomers of the F2-F1 heterodimer assemble to form a mature F protein, which adopts a metastable prefusion conformation that is triggered to undergo a conformational change that fuses the viral and target-cell membranes. Due to its obligatory role in RSV entry, the RSV F protein is the target of neutralizing antibodies and the subject of vaccine development; however, like other RSV antigens, prior efforts to develop an RSV F protein-based vaccine have proven unsuccessful.